Everything You Need to Know about Narcissists, Psychopaths, and Abuse - click on this link: http://www.narcissistic-abuse.com/faq1.html
There are two ways to regulate intimate relationships: via intimacy or with romantic jealousy.
Mature partners achieve a workable balance between togetherness and personal autonomy. They engender intimacy via both communication and actions. Intimacy feels so good that no one in his right mind will exit such a bond. Thus, both abandonment or separation anxiety and romantic jealousy are absent in healthy relationships: object permanence (constancy) is accomplished.
When one of the partners is a codependent or a borderline and the other is a narcissist or a psychopath or a histrionic (common combinations), or when both partners suffer from mood disorders, they cannot achieve or maintain even minimal intimacy. Instead, they mesh, merge, or fuse.
This fusion fosters intolerable abandonment or separation anxiety. The only way to regulate this anxiety is to make sure that the partner doesn't jump ship. This is done by provoking his/her romantic jealousy (a reaction to anticipated loss). Indiscreet extramarital affairs, flagrant promiscuity, ostentatious flirtatious or seductive behaviors with strangers, provocative speech or dress, and hints of impending breakup - are all ways to provoke the partner into sitting up and paying attention and to raise her/his "value" in his/her eyes ("I am desirable and irresistible to other men/women"). The instigator wants her counterpart to set boundaries and put his foot down as proofs that he cares.
Alas, such behaviors precipitate what they had been meant to prevent and have the exact opposite effect to the one intended: anticipating loss and pain aversion combine to drive the injured party away and actually guarantee eventual separation and abandonment.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 3 DVDs with 16 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)